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Sex differences - not as universal as previously thought

In capuchin monkeys, as in many species, males are larger than
females. Are males also more bold, more explorative and less anxious than
females? Far from it. A new study revealed that capuchin monkeys show hardly any sex differences in their individual
behaviours. These findings shed new light on an age-old question.

Capuchins are known for their remarkable cognitive abilities. Now an international research team led by Jana Uher (Primate Personality Net & Free University Berlin), Elsa Addessi and Elisabetta Visalberghi
(ISTC- CNR Laboratory of Cognitive Primatology,
Rome, Italy) comprehensively explored individual differences in the
capuchins' behaviours for the first time.

In the Centro Primati in
Rome, 26 adult tufted capuchin monkeys were studied using 15 different behavioural tests in
which, for example, the monkey individuals could explore novel
objects, manipulate various apparati, or interact with humans or
conspecifics. In addition, the monkeys were observed prior to their main feeding and during their daily activities in their
groups.

The behavioural tests were captured on video and coded later using a special coding software; the observations were recorded
using computerised methods. Overall, 146 behavioural variables were obtained. This meticulous and comprehensive recording of the
monkeys' individual behaviours in various situations enabled detailed and illuminative
analyses.

Importantly, all behavioural tests and observations were
conducted repeatedly and in two nonoverlapping periods of 10 days
each. Overall, the behaviours of a single monkey were recorded for
31.2 hours. These comprehensive data sets allowed the researchers to
analyse whether individual differences occurred only by chance -
after all, like us, monkeys can have a good day or a bad one - or if
the monkeys, in fact, show stable individual differences; that is,
behavioural patterns that are specific to them as individuals. This
proof of stability over some time is essential because only
individual-specific patterns are referred to as
"personality".

So far, "personality" differences have largely been
studied via the use of human's everyday language-the majority of
research on human individual differences is based on the
person-descriptive words that are catalogued in our lexica and, in
particular, in "personality" questionnaires. These methods
can explore what people think about themselves and how they describe
other human individuals or individuals of other species - but these
methods cannot explore how individuals actually behave.

The capuchin monkey study was therefore based on a new research
paradigm developed by Jana Uher to explore and categorise
"personality" differences independent of human's everyday
language. The paradigm also comprises novel methodologies and
approaches that are needed to systematically explore and categorise
individual-specific behaviours not only in humans, but in nonhuman
species as well. The study explains and demonstrates the application
of this novel paradigm and the behavioural research
methodologies.

The results were intriguing. They showed that capuchin monkeys
indeed show stable individual differences across a broad range of
behaviours. The monkeys differed from one another not only in their overall behavioural
tendencies but also in the particular situations in which they showed a particular behaviour in particularly pronounced
ways.

There were some individuals who, of their own initiative, greeted their human observers in their capuchin-specific manners and tried to contact these
humans, whereas other capuchin individuals did so only if the human observers themselves tried to establish contact with them. Other capuchin
monkeys, in turn, approached their human observers only when they were given food. This situation-
specificity of individual behaviour is well documented in humans and also in great
apes. It constitutes an important component of the enormous diversity in which individuality becomes
apparent.

Stunning and unexpected was the finding that, although males are
larger and heavier than
females, as is the case in many other species, capuchin monkeys did not exhibit sex differences in their behaviours except in aggressiveness and
dominance. Instead, there were pronounced individual differences in both
sexes.

Amongst both the males and the females, there were individuals who closely inspected a large bed sheet
that was about 20 times larger than these monkeys and hanging between two horizontal poles, one much higher than the
other. The male Sandokan quickly started to explore the sheet in detail
and even used it as a slide many times. In contrast, Vispo, another male of his group, tried all possible ways to avoid any contact
with the sheet; in fact, he started walking bipedally when he moved over the
poles! As is the case for humans, there were all possible intermediate
variations of behaviour between these two extremes in both males and females.

These new results question assumptions about the universality of sex
differences. They show that, in group-living species led by a dominant male, typically male and typically female tendencies in individual behaviours
do not necessarily have to occur despite pronounced sex differences in body size.

Sex differences in central "personality" characteristics play an important role in many theories of human
evolution, especially with regard to living together and the partition of labour between men and women in social
communities. However, "personality" differences in humans have been studied
thus far almost exclusively with assessments that have employed
standardised questionnaires.

But our everyday language is shaped by
sociocultural perspectives that unintentionally influence our judgements of individual
behaviours. In everyday life, we judge the same behaviours differently depending on whether they are displayed by a male or a
female. Therefore, questionnaire assessments are not suitable for
studying differences in individual behaviours between men and
women. Questionnaire methods are frequently applied because they are efficient and easy to
use, but they very likely constitute an important source of error that has been greatly underestimated so far.

Moreover, it is well-known that cultural beliefs about typically male and female behaviours-that
is, sex and gender stereotypes-influence and change individuals' behaviours in socio-culturally desirable
ways. Many studies have shown that children learn to adopt the particular beliefs of their specific sociocultural community and to behave accordingly only
over the course of their
development.

It may be well possible that differences between males and females are much less rooted in human biology than previously
thought. The new study on capuchin monkeys at least shows that pronounced sex differences in body size need not in general go along with many differences in
males' and females' behaviours as often assumed so far.

Interestingly, there were hardly any age differences in the many
behaviours studied either; older capuchin monkeys behaved only a bit less impulsively than younger
ones. Although all the monkeys were adults, their age range from 8 and 33 years is quite substantial. The analyses also showed that older capuchin monkeys were not more stable in their individual behaviours than younger
ones. Rather, there were pronounced individual differences.

Amongst both the younger and the older
capuchins, some individuals showed very stable individual behavioural tendencies,
thus rendering their behaviours more predictable for human observers than was possible for the behaviours of individuals who behaved very differently from
day-to-day. Such differences in the degree of consistency in individual
behaviours are also well-known in humans and in the great
apes. They constitute a further component of the diversity in which individuality can become
apparent.

Finally, the researchers explored the impact of the capuchins' early life experiences on their individual behaviour as
adults. Monkeys who had to be taken care of by humans in their first year of
life were less aggressive toward human
observers, more distractible by humans, and spent less time close to their conspecifics than mother-reared
monkeys.

These results are remarkable because all monkeys were frequently brought in contact with their conspecifics during their first years of life and could
already be successfully introduced into a group at the age of one
year. This means that all of the capuchin monkeys had been living together with conspecifics continuously for at least 7
years, some even for 32 years. But still, their early life experiences had a significant impact on their behaviours as
adults. These results show how long-lasting the effects of the hand rearing of primate babies in zoological institutions can be on both the
individuals' social behaviours towards humans and towards their
conspecifics.